An auditory prosthesis can be placed behind the ear to deliver a stimulus in the form of an electrical signal to the cochlea of a recipient. These types of auditory prosthesis are generally referred to as cochlear implants. The auditory prosthesis receives sound via a microphone located on a behind-the-ear (BTE) device. The sound is processed and converted to electrical signals, which are sent to an external portion of the auditory prosthesis. The external portion is secured to the head with a magnet that interacts with an implanted magnet in an implanted portion in the recipient's head. Electrical signals are delivered to a coil of the implanted portion from a coil of the external portion. Typically, the implanted magnet is at least partially exposed to the implantation site. This is desirable because removal of the magnet is required if the recipient undergoes, e.g., an MRI procedure. It has been discovered that biofilms (clusters or communities of bacteria that can form on an implanted device and lead to infection) can form on the magnet, generally proximate the interface with the implant itself.